Ties with the European Union are a key issue for Turks long-promised membership of the bloc.
Ties with the European Union are a key issue for Turks long-promised membership of the bloc and Erdogan has rounded savagely on Brussels at the slightest criticism of his actions.
Europe was a "rotting continent ... no longer a centre of democracy, human rights and liberty but of repression, violence and Nazism," he said Thursday.
As for Turkey's EU membership bid, it would be back "on the table" after the referendum, he said Sunday.
Analysts say Erdogan's stinging rhetoric is meant to appeal to Turkish nationalists whose votes will be crucial in Sunday's referendum which looks too close to call.
The hopeful flip side is that he is at heart a pragmatist who will come to terms with the EU when the dust settles.
For the moment however, things are about as bad as they get.
"EU-Turkey relations have never been smooth sailing but the recent exchanges mark a new low," said Jean Marcou, professor at Sciences Po university in Grenoble.
"You really cannot be sure that the relationship will even survive over the next few months," Marcou told AFP.
The Nazi jibes in particular seem a new departure as Ankara lashed out after the cancellation in several EU member states of pro-Erdogan referendum rallies.
"That was an outrage too far because to reopen that wound is the very worst thing you can do in Europe," said Marc Pierini, a former EU ambassador to Ankara.
"They have really burned their bridges when it comes to personal relations," according to Pierini, now an analyst with the Carnegie Europe think tank in Brussels.
At the same time, analysts said, the EU and strategically-placed Turkey still had many shared interests as major trading and investment partners.
If Ankara has opened a door to Moscow, it also remains a key NATO ally at a time when EU leaders are trying to come to terms with a more assertive Russia and the prospect of life without Britain, up to now the bloc's most powerful military member state.
Turkey has also provided safe haven for nearly three million Syrian refugees and a key March 2016 accord has helped stem massive migrant inflows into the EU.
Despite threats to rip it up and open the floodgates again, "Turkey also has its own interest in maintaining the accord," said Marcou, citing EU aid of 3.0 billion euros a year tied to its implementation.
Pierini noted the wider economic ties binding the two sides. "The economic component of the relationship is very substantial."
Against this backdrop, it is hard to overstate the importance of the referendum for both sides, analysts said.
If Erdogan wins, an outright break appears inevitable.
"You would have a single-person system without much rule of law and countervailing power centres," Pierini said.
"It would be authoritarian and clearly in contradiction with European political norms," he added.
As for a 'No' or even a very close 'Yes' vote, that might offer some hope.
"The optimistic scenario now is that a 'No' vote, or potentially even a narrow 'Yes' vote, might lead the Turkish president to reconsider his combative attitude towards Europe and attempt to repair the relationship," said Asli Aydintasbas, senior policy fellow with the European Council on Foreign Relations in Brussels.
"That would require some progress in Turkey's dire human rights situation but Erdogan has been known to show surprising amounts of pragmatism when least expected," Aydintasbas said.